1. Field of the Invention
The present invention relates to a word-length conversion apparatus for converting the word length of digital data. The present invention also relates to a data processing apparatus for carrying out predetermined processing on digital data by performing processing of the digital data along with predetermined processing data.
2. Description of the Related Art
As a technique for converting an audio signal into a digital signal, a DELTA-SIGMA method draws attention. With the DELTA-SIGMA method, a sampling output of an analog audio signal and a previous conversion output are processed and coded in order to convert the analog audio data into digital audio data with a word length of typically 1 bit.
By the way, in some cases, it is necessary to carry out level-adjustment processing even on 1-bit digital audio data obtained by using the DELTA-SIGMA method much like, for example, 16-bit digital audio data resulting from the ordinary linear quantization or the like.
As a technique for adjusting the level of 1-bit digital audio data, a method whereby the 1-bit digital audio data is multiplied by a level-adjustment coefficient (or level-adjustment data) is known as is the case with the adjustment of the level of 16-bit digital audio data.
In such a configuration, however, there is encountered a problem that the accuracy of digital audio data completing level adjustment is much deteriorated.
That is to say, when digital audio data is multiplied by a level-adjustment coefficient, the word length of the digital audio data is increased by a carry. For this reason, it is necessary to truncate as many low-order bits as bits representing the change in word length in order to restore the word length of the digital audio data to the original word length.
When the low-order bits are truncated, however, the accuracy of the digital audio data is deteriorated. In the case of 16-bit digital audio data, the deterioration of the accuracy is not a problem because the number of bits is large.
In the case of 1-bit digital audio data, on the other hand, since the number of bits is small, the accuracy is degraded substantially by the truncation of low-order bits. When 1-bit digital audio data is multiplied by a 16-bit level-adjustment coefficient, for example, it is necessary to truncate 16 low-order bits of the result of the multiplication. As a result, the accuracy of the data completing the level adjustment is considerably deteriorated.
It should be noted that this problem arises not only when level-adjustment processing is carried out on 1-bit digital audio data, but also when processing to eliminate a direct-current offset component or processing to change a frequency characteristic is performed. This is because, even when such processing is carried out, data for eliminating a direct-current offset component and data for changing a frequency characteristic must be added to the 1-bit digital audio data.